In recent years, there have been demands for clean energy in view of environmental issues, and also demands for use of DC power sources as vehicle mounted power sources, power sources for large-size tools and the like. To satisfy such demands, a small-size and light-weight secondary battery which can be charged quickly and also can discharge a high current is required. Examples of typical secondary batteries satisfying such demands include a nonaqueous electrolyte secondary battery. In general, in a nonaqueous electrolyte secondary battery, as a negative electrode material, an active material such as, specifically, lithium metal, lithium alloy or the like is used, or a material in which lithium intercalation compound is inserted in carbon as a host material (which is, herein, a material capable of inserting/extracting lithium ions) is used. As an electrolyte, an aprotic organic solvent in which lithium salt such as LiClO4, LiPF6 or the like is dissolved is used.
Specifically, the nonaqueous electrolyte secondary battery includes a negative electrode plate, a positive electrode plate, and a separator. In the negative electrode plate, the negative electrode material is held in a negative electrode current collector, and in the positive electrode, a positive electrode active material (for example, lithium cobalt composite oxide) which electrochemically reacts reversibly with lithium ions is held in a positive electrode current collector. The separator contains an electrolyte, and is interposed between the negative electrode plate and the positive electrode plate to prevent the occurrence of a short-circuit between the negative electrode plate and the positive electrode plate.
As a method for producing such a nonaqueous electrolyte secondary battery, first, each of a positive electrode plate and a negative electrode plate is formed into a thin film sheet or a foil form, and the positive electrode plate and the negative electrode plate are stacked or spirally wound with a separator interposed therebetween, thereby forming an electricity generating element. Next, the electricity generating element is placed in a battery case made of stainless steel, iron plated with nickel, or some other metal such as aluminum or the like, and a nonaqueous electrolyte is injected into the battery case. Thereafter, a lid plate is firmly fixed to the battery case to closely seal the battery case. Thus, a nonaqueous electrolyte secondary battery is assembled.
In general, when a lithium ion secondary battery is overcharged, or an internal short-circuit occurs in a lithium ion secondary battery, heat is generated in the lithium ion secondary battery and a temperature of the lithium ion secondary battery is increased to high temperature. There may be a risk of excessive heating when the lithium ion secondary battery is under high temperature, and it is therefore desired to improve the safety of the battery. Specifically, since in a large size, high output lithium ion secondary battery, excessive heating occurs with increased probability, efforts to improve the safety of the battery, for example, efforts to reduce a possibility of occurrence of excessive heating, and the like, have to be made.
A major cause of excessive heating occurring when a lithium ion secondary battery is left under high temperature is that a positive electrode active material is unstable in a charged state and under high temperature. That is, when a lithium ion secondary battery is in a charged state and under high temperature, oxygen is eliminated from the positive electrode active material (in general, lithium composite oxide) and the eliminated active oxygen reacts with an electrolyte and the like. Due to this reaction, reaction heat is generated, and thus, the temperature of the positive electrode active material is further increased. When the temperature of the positive electrode active material is further increased, further elimination of oxygen from the positive electrode active material is induced, so that reaction of the active oxygen with the electrolyte and the like is more easily caused and reaction heat is easily generated. In this manner, when the temperature of the positive electrode active material is increased to high temperature, active oxygen reacts with the electrolyte and the like and reaction heat is easily generated, and when reaction heat is generated, the temperature of the positive electrode active material is further increased to higher temperature. It is believed that such chain-reaction heat generation causes excessive heating of the lithium ion secondary battery.
The following is a possible reason for the temperature of the lithium ion secondary battery to be increased to high temperature. When a battery becomes in an abnormal state at a time of overcharge or due to the generation of an internal short-circuit and the like, a separator made of polyethylene is melted or contracted, thus causing a short-circuit of a positive electrode and a negative electrode. Due to this short-circuit, a high current flows and, as a result, the temperature is rapidly increased. Upon rapid increase in temperature, excessive heating of the lithium ion secondary battery occurs in the above-described manner.
As means for improving the safety of a lithium ion secondary battery, a method in which heat stability of a positive electrode active material is improved has been proposed. Specifically, part of Co of lithium cobaltate as the positive electrode active material is replaced with some other element such as Al, thereby improving heat stability of lithium cobaltate (Patent Document 1).
As another means for improving heat stability of a lithium ion secondary battery, a method in which an electrical resistance of an active material is increased to suppress the generation of heat of the battery at a time of a short-circuit has been proposed. Specifically, lithium cobalt composite oxide having a resistance coefficient of 1 mΩ·cm or more and 40 mΩ·cm or less when its powder filling density is 3.8 g/cm3 is used as a positive electrode active material, thereby suppressing the generation of heat of the battery at a time of a short-circuit (Patent Document 2).
As still another means for improving heat stability of a lithium ion secondary battery, a method in which a resistive layer having a higher resistance than a resistance of a current collector is provided on a surface of the current collector has been proposed. Specifically, a resistive layer having a resistance value of 0.1 to 100 Ω·cm2 is provided, thereby preventing a flow of high current even when an internal short-circuit occurs (Patent Document 3).
Patent Document 1: Japanese Published Application No. H11-7958
Patent Document 2: Japanese Published Application No. 2001-297763
Patent Document 3: Japanese Published Application No. H10-199574